There has been marked progress in semiconductor manufacturing technology in recent years. Progress in micro-operations, required by high-density integration, is no exception. In order to achieve high-density packaging, heating of the semiconductors has been decreased and device sizes have been reduced, so that operating currents and voltages in the semiconductors have become to be reduced. The importance of measuring low currents to achieve these reductions is increasing. More accuracy is required in measuring such low currents and in the implements used to make such measurements.
Process monitoring in wafer production is no exception to this. In a conventional method of process monitoring, measurement chips called TEGs (test element groups) are formed in the wafers at the same time as the chips are formed, and are measured to examine device characteristics of the wafer. Interface cards for wafer probers (prober interface cards) are interface cards which mediate between the measurement device and the device which carries/positions the wafer being measured. Interface cards are provided with a number of contact needles surrounding a circular hole in the card, for contacting the measurement terminals of the wafer.
FIG. 1 shows a conceptual diagram of a prior art prober interface card, called probe card 1. FIG. 2 is an enlarged diagram which makes the structure of the needle mounting part easily distinguishable. FIG. 3 is a cross section of FIG. 2, through line a--a'. In FIG. 2, needles 11 and 21 contact terminals 32 and 33 on wafer 31. Signals from terminals 32 and 33 on the wafer pass through respective needles 11 and 21, conductive leads, or signal patterns 12 and 22 on probe card 1, and are then connected to the measurement device (not shown in FIG. 1-3) through terminals (not shown in FIG. 1-3).
Patterns 13 and 23 on probe card 1 are guard patterns (side guard patterns). For example, side guard pattern 13 functions in such a way that currents from the outside through insulator 51 do not flow into signal pattern 12. Patterns 14 and 24 in the substrate of FIG. 3 are also guard patterns (lower guard patterns) which lie in parallel below signal patterns 12 and 22 respectively. For example, lower guard pattern 14 functions in such a way that currents do not pass from the reverse side through insulator 51 of probe card 1 and into signal pattern 12. These lower guard patterns 14 and 24 can also be placed on the reverse side of the substrate, as well as inside it.
Ordinarily, side guard pattern 13 forms an active guard, being given the same potential as needle 11, but it may also be fixed at ground potential, so that it acts as a passive guard. Side guard pattern 23 has the same function with respect to needle 21. Furthermore, only the connections of two needles are shown in FIG. 2, but in fact many needles are provided.
In recent years, measurement accuracies at the fA (femtoampere) level have come to be required in these prober interface cards, but it has been difficult to develop probe cards with the required specifications. One of the reasons for this is the problem of dielectric absorption in the needle mounting parts, which are highly densely spaced.
That is, since many needles are arranged in narrow spaces on the needle mounting parts, it is important to consider the magnitude of dielectric absorption of the pathways from the various needles to the lower guard patterns of adjacent needles through the insulator of the substrate. The magnitude of this dielectric absorption has a large effect on the settling time of the measurement.
On the other hand, use of single-channel coaxial probes makes possible fA-order measurements without use of such cards. However, positioning accuracy of the needles of such probes is poor, and the apparatus is large, so there are restrictions on the number of channels which can be implemented. In addition, such devices are comparatively expensive, and in recent years TEGs become more complex and require many contact probes. As the result, it is not fit for the newest TEGs.
The problems of conventional probe cards will be explained in more detail by use of FIG. 4 which shows an equivalent circuit of the needle part of FIG. 3. In FIG. 4, terminal 211 indicates the potential of needle 11 and signal pattern 12 of FIG. 3. Similarly, terminal 221 indicates the potential of needle 21 and signal pattern 22. Terminal 213 indicates the potential of side guard pattern 13 and lower guard pattern 14, and terminal 223 indicates the potential of side guard pattern 23 and lower guard pattern 24. C12 is the capacitance between terminals 211 and 221, C13 the capacitance between terminals 211 and 213, C24 the capacitance between terminals 221 and 223, C14 the capacitance between terminals 211 and 223, C23 the capacitance between terminals 221 and 213, and C34 the capacitance between terminals 213 and 223. In the equivalent circuit of FIG. 4, the resistance parts have been omitted which are present between the terminals.
In the equivalent circuit shown in FIG. 4, when measurement conditions are changed, the voltage between terminals 211 and 221 changes. In order to simplify the explanation, the case shall be considered in which the voltage of terminal 221 is constant. When the voltage of terminal 211 changes, capacitances C12 and C14 are charged. Since the voltage of terminal 213 is kept equal to that of terminal 211 by the measuring device, C13 can be ignored. Moreover, since C34 is not connected to measurement terminals 211 and 221, it is also ignored.
C12 is the capacitance between the needle 11 and signal pattern 12, and needle 21 and signal pattern 22, of FIG. 3. Since air is only material between these conductors, it is charged instantly. However, C14 is the capacitance between signal pattern 11 and needle 12, and side guard pattern 23 and lower guard pattern 24, in FIG. 3. Since shaded parts 101, 102, 103, etc., of insulator 51 of FIG. 3 are interposed, as insulating parts, in the capacitance space formed by these terminals, the following problem arises.
Ordinally, a larger or smaller dielectric absorption property (dielectric excess effect) is present in insulating parts when a voltage is applied. Insulating parts 101, 102, and 103 cause dielectric polarization, since the voltage applied to these electrodes varies, and a current is drawn for a while until the polarization is completed. Therefore, when the current flowing in needle 11 and signal pattern 12 is measured, the current measurement must be delayed until this inflow of current drops, even though a determined voltage is applied to needle 11. That is, as a result of the settling time being long, a long measurement waiting time is necessary. Moreover, in the converse case, in which the voltage is removed, a long measurement waiting time is necessary, since a discharging current flows in the same theory.
For example, in the conventional prober interface card shown in FIG. 3, when the terminal voltage is changed to 100 V and measure its current within femtoampere error, it is often the case that several tens of seconds are required for the value of the current caused by dielectric absorption to fall to the femtoampere level. This is a significant problem from the standpoint of improving the speed of low current measurements.
Accordingly, it is an object of the invention to provide a prober interface card which shortens the settling time when low currents are measured.
It is a further object of the invention to provide an improved prober interface card which (I) prevents capacitance linking through the insulating parts between probing needles and signal patterns, and conductors with different potentials and (ii) exhibits improved dielectric absorption characteristics.